1. Field of the Invention
The present invention relates to a method for producing glycidol through a series of reactions for the preparation of glycerol carbonate from glycerol and the decarboxylation of the glycerol carbonate.
2. Description of the Related Art
Recently, there has been a rapidly increasing demand for and interest in biodiesel as a new renewable energy source that is synthesized from vegetable oils or animal fats. Biodiesel is produced by reacting fatty acids with alcohols. Biodiesel production generates about 10% (w/w) glycerol as a by-product. An approach for high value-added products from cheap glycerol that is currently in oversupply is thermal decomposition of glycerol carbonate as glycerol derivative to synthesize glycidol. Glycidol is used in various applications, for example, cleansing agents in the petrochemical industry, materials for drug delivery, and raw materials for polymers. Glycidol is produced on an industrial scale by two methods. One method is to oxidize allyl alcohol with hydrogen peroxide in the presence of a catalyst. The other method is to treat chloropropanediol with a base. However, these methods are advantageous in terms of production yield but involve complicated purification steps to produce pure glycidol. Another disadvantage of the methods is decomposition of tungsten oxide catalysts during the catalytic reactions, incurring an increase in production cost and leading to the generation of excess wastewater and salts.
FIG. 1 shows a new method for the synthesis of glycidol by thermal decomposition of glycerol carbonate derived from glycerol. Thermal decomposition of glycerol carbonate is performed using a metal salt catalyst and produces carbon dioxide as well as glycidol. According to this method, glycerol carbonate as a glycerol derivative is subjected to decarboxylation to produce glycidol. The use of the biomass-based raw material makes the method worthy of consideration from an economic and environmental viewpoint.
Other methods for the production of glycidol as a high value-added compound from glycerol carbonate as a raw material are shown in FIGS. 2 and 3. Glycerol carbonate is an intermediate prepared by reacting glycerol with a dialkyl carbonate (FIG. 2) or urea (FIG. 3). For example, Japanese Patent Publication Nos. 2009-137938 and 2009-067689 disclose methods for producing glycidol by reacting glycerol with urea in the presence of ZnSO4 as a catalyst to prepare glycerol carbonate as an intermediate, purifying the glycerol carbonate, and using the purified glycerol carbonate as a raw material for glycidol production. The preparation of glycerol carbonate from urea is advantageous in that the urea price is relatively low but has the disadvantage that it is necessary to reduce the internal pressure of a reactor using a vacuum pump in order to remove ammonia as a by-product, a long reaction time of at least 10 hours is consumed, and thin-film distillation is required to separate the catalyst after the reaction.
Another method for preparing glycerol carbonate as an intermediate for glycidol production is known in which DMC reacts with glycerol in the presence of a basic catalyst. It is known that this reaction enables the synthesis of glycerol carbonate in a yield of 80 to 90% in the presence of a homogeneous base, typically CaO or NaOH. However, there exist problems associated with the separation of glycerol carbonate from the catalyst. In view of these problems, studies on heterogeneous catalysts have been consistently conducted (Applied Catalysis A: General 366, (2009), 315-324). To the best of our knowledge, no report has appeared on methods for the production of glycidol by decarboxylation of glycerol carbonate obtained as a result of the reaction of DMC with glycerol.
The reaction for the production of glycidol through decarboxylation of glycerol carbonate takes place in high yield in the presence of an anion that forms a hydrogen bond of moderate strength with glycerol carbonate. However, in the case where catalysts used in the preparation of glycerol carbonate remain in the reactor for glycidol production, reactions occur between the catalysts, which may seriously affect the decarboxylation yield of the glycerol carbonate. In consideration of this problem, Japanese Patent Publication Nos. 2009-137938 and 2009-067689 describe the use of thin-film distillation as a process for purifying glycerol carbonate to overcome the drawbacks of high boiling point and thermal instability. However, the additional process leads to increases in energy consumption and equipment cost.